The Basics of RIP Version 1

RIP does have its limitations but it is still a venerable routing protocol. Its been around since the 1980s, so has stood the test of time and is supported across a wide variety of platforms, including Windows Servers and both Unix workstations and servers. Run out of routers and need to split a subnet in two? You could use an old server with two NICs and have this speak to your network kit using RIP.  Its clear that more serious admins wouldn’t touch it and it can have a bad reputation but it still does have a place in networking. It is very suitable for smaller networks and its ease of configuration make it a good choice for those network admins with little experience.  It is also an excellent choice for teaching network students about routing protocols as a lot of the structure , such as timers, are used in other protocols.

So here is a general summary of RIP.

• All RIP messages are encapsulated in a UDP segment with the source and destination port of 520.

•  The metric that is used by RIP is hop count. A hop count of 1 is a directly connected network. A hop count of 16 is defined as unreachable.  A hop count metric simply counts the am0unt of router hops that is required to reach a network.

• RIP has an administrative distance of 120.

• RIP employs split horizon with poison reverse.  Split horizon is a feature for preventing routing loops in networks. It basically tells the router that when sending updates out of an interface, do not include any networks that were learned from updates received on that same interface. Poison reverse adds an additional feature. When sending updates out of an interface, for any updates learned on that interface the router now marks those networks as unreachable. It’s neighboring router now has positive confirmation that these networks cannot be reached via this source.

RIP Timers

Update Timer

A RIP router sends its entire routing table to its neighbours at regular intervals. This interval is defined using the update timer and by default is set at 30 seconds. The router, however, adds a random variable to the timer to stop the routers’ updates becoming synchronised to each other. A router’s regular update will therefore be between 25 and 35 seconds.

Invalid Timer

The invalid timer is set to 180 seconds whenever a new route is learnt. The timer is reset each time the router receives a update containing that route. If no update is received for a route already in the routing table within 180 seconds, the hop count for that route is changed to 16 (unreachable)

Flush Timer

The flush timer is set at 240 seconds by default. A route marked as unreachable will continue to be advertised by the router until the flush timer has expired. After this time it will be completely removed from the routing table.

Holddown Timer

If a router receives an update that either has a higher or unreachable metric for an active route in the routing table, it will start the holddown timer. The new, less preferred, route entry will not be placed in the table until the holddown timer expires. For RIP the default for the holddown timer is 180 seconds.

RIP Message Format

The RIP protocol defines two RIP message types:

• Request message – used to ask neighbouring routers to send updates
• Response message – carries the updates 

• Each message can contain up to 25 route entires.

• Each entry will contain an Address Family Identifier, the IP address of the route and the metric.

• Command – if set to 1 the message is Request. If set to 2 the message is a Response.

• Addrress Familiy Identifier – is set to 2 for IP.

• Metric – the hop count between 1 and 16.

I hope you will find this article a decent quick study guide about the basic theory of RIP. I hope to do an article at some point on the configuration of RIP.

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